1. Field of the Invention
This invention relates to polycrystalline diamond cutters for use in earth boring bits. Specifically, this invention relates to polycrystalline diamond cutters which have modified substrates to selectively modify and alter residual stress in the cutter structure.
2. Statement of the Art
Polycrystalline diamond compact cutters (hereinafter referred to as “PDC” cutters) are well-known and widely used in drill bit technology as the cutting element of certain drill bits used in core drilling, oil and gas drilling, and the like. Polycrystalline diamond compacts generally comprise a polycrystalline diamond (hereinafter “PCD”) table formed on a carbide substrate by a high temperature-high pressure (hereinafter “HTHP”) sintering process. The PCD table and substrate compact may be attached to an additional or larger (i.e., longer) carbide support by, for example, a brazing process. Alternatively, the PCD table may be formed on an elongated carbide substrate in a sintering process to form the PDC cutter with an integral elongated support. The support of the PDC cutter is then brazed or otherwise attached to a drill bit in a manner which exposes the PCD table to the surface for cutting.
It is known that PDC cutters, by virtue of the materials comprising the PCD table and the support, inherently have residual stresses existing in the compact therebetween, throughout the table and the carbide substrate, and particularly at the interface. That is, the diamond and the carbide have varying coefficients of thermal expansion, elastic moduli and bulk compressibilities such that when the PDC cutter is formed, the diamond and the carbide shrink by different amounts. As a result, the diamond table tends to be in compression while the carbide substrate and/or support tend to be in tension. Fracturing of the PDC cutter can result, often in the interface between the diamond table and the carbide, and/or the cutter may delaminate under the extreme temperatures and forces of drilling.
Various solutions have been suggested in the art for modifying the residual stresses in PDC cutters so that cutter failure is avoided. For example, it has been suggested that configuring the diamond table and/or carbide substrate in a particular way may redistribute the stress such that tension is reduced, as disclosed in U.S. Pat. No. 5,351,772 to Smith and U.S. Pat. No. 4,255,165 to Dennis. Other cutter configurations which address reduced stresses are disclosed in U.S. Pat. No. 5,049,164 to Horton; U.S. Pat. No. 5,176,720 to Martell, et al.; U.S. Pat. No. 5,304,342 to Hall; and U.S. Pat. No. 4,398,952 to Drake (in connection with the formation of roller cutters).
Recent experimental testing has shown that the residual stress state of the diamond table of a PDC cutter can be controlled by novel means not previously disclosed in the literature. That is, results have shown that a wide range of stress states, from high compression through moderate tension, can be imposed on the diamond table by selectively tailoring the carbide substrate. Thus, it would be advantageous in the art to provide a PDC cutter having selectively tailored stress states, and to provide methods for producing such PDC cutters.